Recent advances in high intensity discharge lamp technology are creating a material inventory problem for lighting fixture manufacturers. There are three popular high intensity discharge (HID) lamp types today, mercury vapor, metal halide and high pressure sodium. These lamps are available in various wattage sizes including 35, 50, 70, 75, 100, 150, 175 and others up to 1500 watts. In addition, these types and sizes are made to operate at various voltages including 120 volts, 208 volts, 240 volts, 277 volts, and 480 volts.
Each of these types of lamps requires a ballast in order to operate properly. Each ballast circuit includes a capacitor for efficient and economical operation. This capacitor, or capacitors in some cases, determines the amount of power delivered to the lamp. If more capacitance than that specified is used, the lamp will be forced to operate at a higher wattage than normal. If the capacitance used is lower than the correct value, the lamp will operate at a reduced wattage. Either condition can be harmful to the lamp or the ballast or both. It is therefore imperative for a lighting fixture manufacturer to deal with the numerous variations in types, wattages, and voltages by stocking a variety of capacitor sizes. In this sense, capacitor size is used to refer to electrical size, but, as is well known, capacitors which are made with different values of capacitance and for use with different voltages also usually exhibit different physical sizes as well.
The most popular capacitors used by the lighting industry are packaged in oval cans of which there are several sizes. Commonly, a capacitor is constructed by winding a polypropylene insulator film with a metalized aluminum film to act as an electrode, connecting the appropriate portions together and placing this assembly inside a deep-drawn oval can which is then filled with oil and hermetically sealed. The resulting article can be rather bulky, in sizes usable in lighting circuits. A typical 14 microfarad capacitor rated for 240 volts has a width of 3 7/16 inches, a thickness of 21/4 inches, a height of 41/4 inches and a weight of 61/2 lbs. It is quite important that this capacitor be held firmly in place, but it must not be held too tightly because compressive forces on the exterior of the can can cause oil leakage or physical damage to the can which can also involve electrical damage to the capacitor.
When installed in lighting fixtures, a capacitor must be held in place in any of a variety of mounting surfaces and conditions. Typical situations involve either a cast housing or a sheet metal panel against which the capacitor is to be held. Normally, the housing is provided with two fastener devices which are spaced apart to form a gap between which the capacitor is to be mounted. However, In any one type of housing, a variety of different capacitor sizes may be usable, depending upon the ballast and lamp sizes which are to be installed in that housing. Thus, a variety of clamping brackets are required.
In order to try to minimize the number of different types of clamping brackets which must be produced and stocked in inventory, various techniques have been employed. One example is the use of a metal strip with a series of perforations down the center of the strip. When using this kind of material, a portion of the strip is cut to approximate the length that will reach around the largest capacitor can from one fastener to the other. Depending on the size of the can, the installer will select the combination of two holes in the strip which will most satisfactorily hold the capacitor in place. Obviously, this method rarely if ever will provide a combination of holes which, upon tightening the bolts, will provide the precise length of strap necessary to hold the capacitor without denting the capacitor can. As a practical matter, this method has done more damage than good.